The formation of Nickel based germanosilicides (NiSiGe) has attracted growing interest in the state-of-the-art metal oxide semiconductor field effect transistor (MOSFET) technology, because silicon-germanium alloy (Si1-xGex) is used as embedded source/drain stressor or channel material to enhance the hole mobility in the channel region. However, a major problem of NiSiGe film is that it has a poor thermal stability after annealing at high temperature (550 ℃), which leads to its agglomeration. In this work, we study the reaction between Ni and Si0.7Ge0.3 in the presence of an Al interlayer. Pure Ni (10 nm) film and Ni (10 nm)/Al (3 nm) bi-layers are deposited respectively on Si0.7Ge0.3 substrates by electron beam evaporation. Solid-phase reactions between Ni or Ni/Al and Si0.7Ge0.3 during rapid thermal processing in N2 ambient for 30 s are studied at 700 ℃. The un-reacted metal is subsequently etched in H2SO4 solution. The NiSi0.7Ge0.3 films are characterized by Rutherford backscattering spectrometry (RBS), crosssection transmission electron microscopy (XTEM), energy dispersive X-ray spectrometer (EDX), and secondary ion mass spectroscopy (SIMS) techniques. For the Ni/Si0.7Ge0.3 sample, the segregation of Ge at grain boundaries of nickel germanosilicides during the interfacial reactions of Ni with Si0.7Ge0.3 films and the subsequent formation of Ge-rich Si1-wGew (w0.3) are confirmed by the RBS and XTEM measurements. However, in the case of Al incorporation, a very uniform and smooth NiSi0.7Ge0.3 film is obtained with atomic NiSi0.7Ge0.3/Si0.7Ge0.3 interface. The orthorhombic NiSi0.7Ge0.3 is finally epitaxial grown on cubic Si0.7Ge0.3substrate tilted at a small as demonstrated by the High resolution XTEM. Furthermore, based on the EDX and SIMS measurements, it is found that most of the Al atoms from the original interlayer diffuse towards the NiSi0.7Ge0.3 surface, and finally form an oxide mixture layer. It is proposed that the addition of Al reduce Ni diffusion, balance the Ni/Si0.7Ge0.3 reaction and mediate the NiSi0.7Ge0.3 lattice constant. In addition, the main mechanism of epitaxial growth of NiSi0.7Ge0.3 film is analyzed in detail. In summary, Al mediation is experimentally proved to induce the epitaxial growth of uniform and smooth NiSi0.7Ge0.3 layer on relaxed Si0.7Ge0.3 substrate, providing a potential method of achieving source/drain contact material for SiGe complementary metal oxide semiconductor devices.